Determining a white blood cell count is often important in connection to treating a patient. This analysis may be needed for diagnosing e.g. leukaemia, or infectious or inflammatory diseases or for monitoring treatments. It is desirable to enable analysis results to be obtained as quickly as possible in order to minimize waiting times for patients and enabling a physician to make a decision of treatment and diagnosis directly when making a first examination of the patient. It would therefore be preferable to provide an analysis method which may be quickly performed by the physician or a nurse without the need of sending a test away to a laboratory. Determining the white blood cell count is one of the most common tests being performed on patients in establishing a diagnosis. Therefore, it would be very advantageous to have a quick and simple method of performing the analysis.
Today, a white blood cell count is normally obtained through a manual procedure by staining a blood sample and microscopically viewing the sample in a special counting chamber, e.g. a Bürker chamber. The counting chamber is provided with a grid dividing the chamber in well-defined small volumes. The white blood cells are allowed to settle at the bottom of the counting chamber in order to enable the microscope to focus on all cells in the chamber and, thus, facilitate counting. Thus, the sample needs to settle for several minutes before the counting may be performed. The white blood cell count can then be determined by counting the number of blood cells per box in the grid. The white blood cell count is obtained manually by an analyst, who needs to be experienced in performing the analysis in order to be able to perform a reliable analysis.
This analysis is time-consuming. Further, since it is performed manually, the results of the analysis may vary depending on the person performing the analysis.
There are a few number of existing automated analysis methods for determining a white blood cell count. The white blood cell count may be determined by means of the Coulter principle, which is based on determining cell size and thereby the cell type by sensing an impedance. A method for counting white blood cells by the Coulter principle is described in U.S. Pat. No. 5,262,302. Measurement apparatus according to the Coulter principle is expensive and it is therefore a considerable investment. Thus, a hospital or laboratory will be reluctant to invest in more than one apparatus. This implies that the analysis will need to be performed in a centralised location and a patient will need to wait for analysis results.
The Coulter principle is the dominating, automated analysis method that is presently being used. However, there are a few other methods that have been described. One such method for determining a white blood cell count is disclosed in U.S. Pat. No. 5,585,246. Here, a blood sample has to be prepared by being mixed with a fluorescent dye and ligand complex which tags the white blood cells. The sample is introduced into a capillary and is irradiated by a laser source which scans over the sample in the capillary. The fluorescence is measured in order to determine the number of white blood cells. A similar method is disclosed in WO 97/02482, using a fluorescent dye and a laser source scanning over a capillary. This method is adapted for enumeration of white blood cells in apheresis products containing a low number of white blood cells. Here, the capillary is quite thick and it is necessary to wait until the white blood cells have settled at the bottom of the capillary before the capillary may be scanned.
In WO 99/45384, a sample-containing chamber having varying thickness is shown. The varying thickness separates different compounds of blood. The blood sample is stained with a colorant to differentially highlight at least three different white blood cell types in the blood sample. The white blood cells may be enumerated by using an optical scanning instrument to view a portion of the chamber.
In WO 98/50777, a method for assessment of the number of somatic cells in milk is disclosed. The method comprises applying a volume of a sample in a sample compartment and transmitting electromagnetic signals, having passed from the sample compartment, onto an array of detection elements. The intensities of detected electromagnetic signals are processed and the results are correlated to the number of cells present in the sample.
There is still a need to speed up and simplify existing automated methods for determining a white blood cell count such that the analysis may be performed by any user, not requiring special training, and such that measurement apparatuses may be relatively inexpensive. This would imply that the analysis may be provided at a point of care. Further, since the white blood cell count is such a commonly performed analysis, any improvement to the analysis method would have a positive impact on patient care. An analysis method providing a possibility to obtain results at a point of care would be particularly advantageous.
Also, it may be advantageous to obtain a differential white blood cell count, that is to examine the distribution of different types of white blood cells in a blood sample. This differential white blood cell count may reveal if the cells are present in a normal distribution, or if any cell type is increased or decreased. The information may be useful in diagnosing specific types of illness. For example, an increase in neutrophils indicates a bacterial infection, whereas an increase in lymphocytes is common in acute viral infections.
The differential white blood cell count may also be obtained by microscopically viewing and manually counting stained blood cells in a Bürker chamber. There also exist some automated methods. For example, a differential count may be obtained with the Coulter principle by analysing the form and size of the electrical pulse generated by a cell passing through an electrical field. The form and size of the pulse may be related to the type of white blood cell being detected. One such method is described in U.S. Pat. No. 4,528,274.
In U.S. Pat. No. 5,123,055, another method for identifying different types of white blood cells is described. This method requires several size and colour parameters to be sequentially analysed in order to differentiate the types of white blood cells.
It is still desired to speed up and simplify existing automated methods for determining a differential white blood cell count. It would be particularly advantageous to provide a quick, simple and relatively inexpensive analysis method such that the analysis may be provided at a point of care.